A wireless local access network (WLAN) is shown in FIG. 1. An access point (AP) is responsible for performing bidirectional communication with multiple stations (STA). That is, the AP sends downlink data to STAs (for example, a STA 1 and a STA 2 in FIG. 1) or receives uplink data from STAs (for example, a STA 3 and a STA 4 in FIG. 1).
A WLAN standard based on an orthogonal frequency division multiplexing (OFDM) technology appears in releases including 802.11a, 802.11n, 802.11ac, and the like. A WLAN device (an AP or a STA) obtains, by means of carrier sense multiple access (CSMA) contention, a right of using a channel, that is, first performing clear channel assessment (CCA) detection before sending data. Specifically, before sending data by using a channel, the WLAN device first receives a signal on the channel in a period of time. When an average power of the received signal exceeds a specified threshold or the received signal satisfies a period requirement, the WLAN device determines that the channel has been occupied by another device. When the average power of the received signal does not exceed a specified threshold or the received signal does not satisfy a period requirement, the WLAN device determines that the channel is in an idle state, and starts transmitting data by using the channel.
In an existing WLAN, a specific data transmission process may be a case shown in FIG. 2. In the data transmission process, an AP starts performing CCA detection on a channel at a moment t1. When determining that the channel is idle, the AP starts sending a downlink data frame to a STA 1 at a moment t2, and finishes sending the downlink data frame at a moment t3. The STA 1 receives the downlink data frame in a corresponding time. After a short inter-frame interval (SIFS) time, if the STA 1 correctly receives the downlink data frame, the STA 1 sends an acknowledgement (ACK) frame or a block acknowledgement (BA) frame to the AP at a moment t4. The AP determines, by receiving the ACK/BA frame sent by the STA 1, that the downlink data frame has been correctly received by the STA 1, thereby finishing the current downlink data transmission operation, and releases the right of using the channel. Similarly, when a STA 2 needs to send uplink data to the AP, the STA 2 performs CCA detection at a moment t5. If determining that the channel is idle, the STA 2 starts sending an uplink data frame to the AP at a moment to, and finishes sending the uplink data frame at a moment t7. After an SIFS time, if the AP correctly receives the uplink data frame, the AP sends an ACK/BA frame to the STA 2 at a moment t8. After receiving the ACK/BA frame, the STA 2 finishes the current uplink data transmission operation, and releases the right of using the channel.
In a new WLAN standard 802.11ax based on orthogonal frequency division multiple access (OFDMA), an AP may divide an entire frequency band into multiple frequency bands in an OFDMA manner, and separately send downlink data to multiple STAs simultaneously, or simultaneously receive uplink data from multiple STAs in an OFDMA manner. In a WLAN system based on OFDMA, to be compatible with an existing WLAN device, the AP still contends for a channel by means of CSMA, that is, first performing CCA detection on a channel before using the channel. If determining that the channel is idle, the AP contends for the channel. If the contention is successful, the AP occupies, for a period of time, a transmission opportunity (TXOP) for uplink or downlink transmission, or uplink and downlink cascaded transmission. If the AP needs to transmit downlink data to at least one STA, similar to the AP in the WLAN based on CSMA, the AP directly sends the downlink data frame after determining, by means of CCA detection, that the channel is idle. Multiple STAs may be multiplexed in an OFDMA manner to perform transmission. Different from the WLAN based on CSMA, STAs do not directly initiate uplink transmission by means of channel contention. Instead, STAs are scheduled to perform uplink transmission after the AP contends for the channel.
As shown in FIG. 3, when scheduling a STA 2 and a STA 3 to transmit uplink data in an OFDMA manner, an AP sends a trigger frame after obtaining, by means of CCA detection, a right of using a channel. The trigger frame indicates the STA 2 and the STA 3 that are scheduled and resources used by the STA 2 and the STA 3 to transmit the uplink data. As shown in FIG. 3, the AP sends the trigger frame at a moment t6. Then, after an SIFS time, the STA 2 and the STA 3 that are scheduled start separately sending an uplink data frame at a moment t7 by using the resources allocated by the AP. If correctly receiving the uplink data sent by the STA 2 and the STA 3, the AP finishes the current uplink transmission process after sending an ACK/BA frame. During uplink transmission, a TXOP reserved by the AP includes at least a time from the beginning of sending of the trigger frame to the end of sending of the ACK/BA frame.
The WLAN standards such as 802.11n, 802.11ac, and 802.11ax all support transmission of a multiple input and multiple output (MIMO) system. That is, multiple spatial flows are transmitted together on a same channel to achieve a higher data throughput, or each STA uses one or more spatial flows, to achieve an objective of serving more users. In this case, a transceiver of the WLAN includes at least two transmit paths and at least two receive paths. Due to limitations such as complexity and costs, a maximum path bandwidth supported by the WLAN standards including 802.11ac and 802.11ax is 160 megahertz (MHz), and a maximum quantity of spatial flows supported is 8. However, most actual WLAN devices support a maximum bandwidth of only 80 MHz.
In the WLAN system, transmission and reception in parallel (TRIP) is a solution in which a channel for uplink transmission and another channel for downlink transmission in the WLAN are simultaneously used to perform parallel transmission, so as to improve a throughput at an AP end. In the TRIP solution, although each channel may perform uplink or downlink transmission in different times, when a first channel sends data, a second channel only receives data, and when a first channel receives data, a second channel only sends data. In this way, for a WLAN device supporting transmission and reception in parallel, if a transceiver of the WLAN device includes m transmit paths and n receive paths, where m≥2, and n≥2, data may be sent on the first channel by using all the transmit paths (in transmit paths), and simultaneously, data is received on the second channel by using all the receive paths (n receive paths); or data is sent on the second channel by using all the transmit paths (m transmit paths), and simultaneously, data is received on the first channel by using all the receive paths (n receive paths). That is, a system throughput may be effectively improved by fully using processing capabilities of the existing transmit and receive paths without increasing complexity (including path bandwidths and a quantity of paths) of the transmit paths and the receive paths.
FIG. 4 shows some typical application scenarios of a TRIP solution. In FIG. 4(a), only an AP supports uplink and downlink parallel transmission while STAs do not need to support uplink and downlink parallel transmission. The AP sends downlink data to a STA 1 by using a first channel whose carrier frequency is f01, and simultaneously receives, by using a second channel whose carrier frequency is f02, uplink data sent by a STA 2. There are at least an interval of 100 MHz between a channel frequency band of the carrier frequency f01 and a channel frequency band of f02. Therefore, receiving and sending of the two channels do not interfere with each other. FIG. 4(b) is similar to FIG. 4(a). Only an AP supports uplink and downlink parallel transmission while STAs do not need to support uplink and downlink parallel transmission. The AP sends downlink data to a STA 1 and a STA 2 by using a first channel whose carrier frequency is f01, and simultaneously receives, by using a second channel whose carrier frequency is f02, uplink data sent by a STA 3 and a STA 4. The STA 1 and the STA 2 may be multiplexed on the first channel by means of OFDMA and/or downlink multi-user MIMO (MU-MIMO), and the STA 3 and the STA 4 may be multiplexed on the second channel by means of OFDMA and/or uplink MU-MIMO. In FIG. 4(c), both an AP and a STA 3 support uplink and downlink parallel transmission. The AP sends downlink data to the STA 3 by using a first channel whose carrier frequency is f01, and simultaneously receives, by using a second channel whose carrier frequency is f02, uplink data sent by the STA 3. FIG. 4(d) is similar to FIG. 4(c). An AP, a STA 1, and a STA 2 all support uplink and downlink parallel transmission, but a STA 3 and a STA 4 do not support uplink and downlink parallel transmission. The AP sends downlink data to the STA 1, the STA 2, and the STA 4 by using a first channel whose carrier frequency is f01, and simultaneously receives, by using a second channel whose carrier frequency is f02, uplink data sent by the STA 1, the STA 2, and the STA 3.
FIG. 5 shows a typical data transmission process of TRIP in the prior art. A scenario of this embodiment is shown in FIG. 4(a) and FIG. 4(b). Only an AP performs uplink and downlink parallel transmission while STAs do not perform uplink and downlink parallel transmission. Using the scenario shown in FIG. 4(a) as an example, the AP sends downlink data to the STA 1 by using the first channel whose carrier frequency is f01, and simultaneously receives, by using the second channel whose carrier frequency is f02, uplink data sent by the STA 2. As shown in FIG. 5, the AP performs, at a moment t1, CCA detection on the first channel by using at least one but no more than n−1 receive paths, and performs CCA detection on the second channel by using remaining at least one receive path. If both the first channel and the second channel are idle, the AP reserves a TXOP for uplink and downlink parallel transmission. After an SIFS time, the AP sends a first trigger frame on the first channel at a moment t2 by using at least one but no more than m−1 transmit paths, to send downlink transmission scheduling control information, and simultaneously sends a second trigger frame on the second channel by using remaining at least one transmit path, to send uplink transmission scheduling control information. The scheduling control information includes, but is not limited to, information such as identifiers of STAs that perform uplink or downlink data transmission after a trigger frame, transmission resources (for example, subcarrier resourced of a frequency domain) used by the STAs to perform data transmission, a quantity of spatial flows and corresponding identifiers of the spatial flows, a modulation coding scheme (MCS) used to transmit a corresponding spatial flow, and the like. After the SIFS time, the AP may send a downlink data frame on the first channel at a moment t3 by using all the m transmit paths. At the same time, the STA 2 sends, at the moment t3 according to the uplink transmission scheduling control information sent by the second trigger frame, an uplink data frame to the AP on the second channel. The AP may receive, on the second channel, the uplink data frame by using all then receive paths. In the SIFS time after transmission of the uplink and downlink data frames is finished, if the STA 1 correctly receives the downlink data frame sent by the AP, the STA 1 sends an uplink ACK/BA frame to the AP at a moment t4. Moreover, if the AP correctly receives the uplink data frame sent by the STA 2, the AP also sends a downlink ACK/BA frame to the STA 1 at the moment t4. In an ACK/BA frame transmission phase, the AP may send, on the second channel, the downlink ACK/BA frame by using all the m transmit paths, or may receive, on the first channel, the uplink ACK/BA frame by using all the n receive paths. Lengths of the uplink ACK/BA frame and the downlink the ACK/BA frame may be different. The TXOP reserved by the AP should include a time from the moment t2 when the trigger frame is sent to a moment when transmission of a relatively long frame in the uplink and downlink ACK/BA frames is finished.
In conclusion, in the TRIP solution, all receive paths and transmit paths on an AP can be fully used for transmission, and a throughput of the AP is improved by using one of dual channels for receiving and the other for sending. However, the TRIP solution has the following conditions in application:
In a frequency band at which an AP works, the AP needs to find two idle channels to perform receiving and sending.
Both a receive path and a transmit path of the AP need to be in an idle state.
According to a core idea of TRIP, if a STA occupies, in the uplink or downlink, an operating channel and receive and transmit paths of the AP by means of CSMA, the AP may schedule, when the other channel is idle, a service in an opposite direction to occupy the channel. For example, when the STA performs uplink transmission by means of CSMA, the AP provides a downlink transmission service on the other channel. However, an uplink transmission time of the STA is not necessarily known at an AP end when transmission begins. If the AP performs scheduling and transmission according to a downlink transmission requirement of the AP without knowing the uplink transmission time, a case in which uplink transmission is finished while downlink transmission has not finished may occur. In this case, the downlink transmission at the AP end occupies all or most of transmit paths. Therefore, the AP can perform only uplink receiving on an idle channel on which the uplink transmission is finished. The cycle repeats. Due to a misaligned state between the uplink and downlink, one channel of the AP keeps performing uplink transmission, while the other channel keeps performing downlink transmission. Consequently, the dual channels are locked. As a result, in the channel keeping performing uplink transmission, a downlink service times out, while in the channel keeping performing downlink transmission, an uplink service also times out. Consequently, a principle of contention fairness of the WLAN is violated.